National Weather Service Raw Text Product

870 
FXUS62 KGSP 240751
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
351 AM EDT Tue Sep 24 2024

.SYNOPSIS...
A moist and cooler airmass will remain in place across the forecast 
area through mid-week. What is expected to become Hurricane Helene 
is forecast to make landfall in the Florida panhandle late Thursday, 
with the remnant circulation moving quickly north and bringing wind 
and heavy rainfall impacts to our area Thursday night into Friday. 
Calmer and seasonably warm conditions are expected over the 
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 3:25 AM EDT Tuesday: We've still got a few patches of showers
making their way eastward over western NC early this morning. They
have yet to produce any lightning, but we could see a strike or two 
before they dissipate. As anticipated, a layer of low stratus has 
spread over much of the fcst area over the past couple of hrs, with
a handful of sites reporting patchy fog. If more sites begin repor-
ting fog/reduced visby, an SPS may be needed. With the increased 
cloud cover, low temps should remain about 10 degrees above normal. 

Otherwise, today will be similar to yesterday (Monday) with morning 
low clouds slowly eroding and lifting from SW to NE thru the morning
and into the afternoon. Low pressure will develop to our NW over the
Ohio River Valley and help keep a broad area of moist air in place
over the region. Moderate instability along with 30 to 45 kts of deep-
layer shear can be expected across our area during the afternoon and 
evening. Expect sct convective coverage during the afternoon with the 
best coverage more likely during the evening hrs. The severe potential 
looks better for our area today/tonight with SPC's current day 1 Con-
vective Outlook having most of our CWA under a Marginal Risk for svr 
thunderstorms. The main threats will be damaging wind gusts and hail. 
Otherwise, high temps will top-out roughly 3 to 6 degrees above normal 
with our SW zones approaching 90 degrees again.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
As of 245 am Tuesday: A strong upper low is forecast to be centered 
near the confluence of the Ohio and Mississippi Rivers at the start 
of the period, slowly wobbling south through Thursday, as what is 
expected to become Hurricane Helene makes landfall in the Florida 
panhandle Thu evening. Throughout Wednesday, an inverted surface 
trough is forecast to organize across the region...most likely 
centered across the TN Valley, while increasing southerly mid/upper 
level flow on the east side of the upper low begins to draw a plume 
of tropical moisture from Helene into the region. The interaction of 
this plume with increasingly deep low level convergence in the 
vicinity of the surface trough. This could result in evolution of a 
heavy predecessor rainfall event (PRE) from Wed into Wed night, 
which may incorporate much of southwest NC, eventually including an 
upslope component as southeast flow accelerates above the surface 
layer late Wed through early Thu. Whether the axis of heavy rain 
sets up within, or just west of our forecast area is uncertain at 
this juncture...it just needs to be noted that widespread amounts of 
at least a couple of inches could accumulate across a portion of the 
area before direct impacts from Helene arrive. Regardless, PoPs will 
increase throughout Wed as spokes of vorticity rotating around the 
upper low approach the forecast area and interact with a moderately 
unstable and increasingly moist environment. 

Other than the potential for lingering upslope showers on Thursday, 
there may be a bit of a lull in precipitation through much of 
Thursday, as inverted surface trough shifts well west of the 
Appalachians. Meanwhile, Helene is expected to lift quickly N/NW 
through Georgia Thursday night, with the center likely passing near 
the intersection of  GA/TN/NC Friday morning. This will place at 
least the western part of the forecast area on the favorable right 
side of the cyclone track for heavy tropical rain bands. Heavy 
rainfall may pile up quickly Thu night, with excessive rainfall and 
at least isolated flash flooding becoming likely, especially from 
northeast GA and the Upstate into southwest NC. By noon Friday, 
total rainfall amounts (including anything falling prior to Helene's 
arrival) are expected to range from 4 to 7 inches across much of the 
area west of ~I-77. Locally higher amounts as high as a foot are 
expected across southwest NC...especially along the Blue Ridge 
escarpment. 

Confidence in winds and wind gusts remains modest at best. However, 
a consensus of the latest short term guidance suggests Helene will 
still be of solid tropical storm intensity as it passes near the CWA 
Thu night. Our current best guess is that much of the western half 
of the area will see plenty of 35-50 mph gusts, with damaging wind 
gusts very much a possibility just to the east of the cyclone's 
center. Being on the right side of the cyclone also poses some 
concern for isolated tornadoes, as shear parameters will be more 
than adequate for rotating cells within rainbands. However, 
tornadoes tend to have a strong diurnal correlation in association 
with inland tropical cyclones, and guidance suggests surface-based 
instability will be largely absent by the time rainbands arrive Thu 
night, so the tornado threat appears to be small (but non-zero) at 
this juncture. Otherwise, wind and rain are expected to steadily 
diminish throughout Fri morning, with quiet weather returning by 
evening.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
As of 335 am EDT Tuesday: An upper low is still expected to be 
wobbling near Memphis at the start of the extended, but should 
finally weaken and be nudged into the eastern Conus early next week 
as a strong short wave trough grazes the north central part of the 
country. After a brief lull in significant precip chances over the 
weekend, PoPs begin ramping up again early next week, albeit 
remaining in the chance range. Temperatures begin the period close 
to normal, but fall below normal early next week, when global model 
guidance suggests development of cooling E/NE flow in association 
with an inverted surface ridge/potential cold air damming.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Patches of sct showers will likely linger
thru the overnight hrs, especially across western North Carolina. 
This was handled with VCSH at the NC terminals for the next few 
hrs. We may need to extend the VCSH if shower activity continues 
for longer. Winds should remain light to calm across the area thru
much of the morning. Another round of IFR to LIFR low stratus is 
expected overnight and thru the morning, with the lower cigs likely
lingering into the early afternoon for KCLT and KHKY, but probably
not quite as long for the Upstate terminals or KAVL. There's a de-
cent chance that patchy fog will develop as well but confidence is
not as high as it is for lower cigs, so any IFR/LIFR visby was han-
dled with TEMPOs. Fog should mix out by 14z or so, with the lower
cigs likely to linger a few hrs longer. Another round of sct showers
and thunderstorms is expected this aftn/evening, so I have PROB30s 
for TSRA at all taf sites from roughly 18 to 24z with TEMPOs for 
lingering showers after 00z. Winds will pick up modestly this aftn 
from the S to SW, and become light again later this evening.  

Outlook: A slowly approaching weak cold front will stall west of 
the fcst area on Wednesday, keeping the chance for diurnal showers 
and thunderstorms around. Fog/low stratus may develop each morning, 
especially in the mtn valleys. Confidence is increasing that a 
tropical cyclone will bring heavy rain and strong winds Thursday 
into Friday before moving out of the area Friday night into Sat.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$

SYNOPSIS...JDL
NEAR TERM...JPT
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...JPT